With fuel shortage problems, solar heating of water is becoming more important.
There are many countries in the world where solar heating can be advantageously applied such as for household storage systems and the like but problems in the past have been to develop systems wherein good heat transfer resulted and where problems, such as contamination due to sedimentation are minimised.
A customary method of heating, for instance, the water for a household system has been to place a storage container into the roof of the building or at any other suitable locality and to connect the water from this to a solar device comprising a series of tubes through which the water flows by thermo-siphon or pumping action, which tubes are positioned on a roof or the like facing the sun, and are protected against loss of heat by enclosing the tubes in a container or panel having a glass face through which the sunlight reaches the tubes containing the water.
One of the problems with this type of assembly is to apply the heat so collected to the walls of the tubes, and more difficult still to apply the heat efficiently to the water. It has been proposed to use fins between the tubes to increase the area of heat receptive material. It has also been proposed to extrude black plastic members but plastics are bad heat conductors.
An alternative form has been to use clear tubes and to have in these a black fluid such as oil which was highly receptive of heat but this necessitated a heat exchanger within the storage means to transfer the heat from the oil to the water.
According to another form shown in the specification of U.S. Pat. No. 1,889,238 an opaque absorber sheet was placed behind a glass wall to reduce heat loss, and the back of the absorber sheet was in contact with the water so that heat from the absorber sheet was transferred to the water but from the back of the absorber sheet.
According to another form shown in the specification of U.S. Pat. No. 3,250,269 the water was confined between a front glass or plastic cover and a back reflective surface which reflected rays passing through the water back through the water. The rays thus passed twice through the water but then passed out through the front cover as reflection does not change the wavelength. While the back of the reflector was painted black, that surface had minimal absorption because of the highly reflective front surface.